Manufacture of abrasive articles



i atente Apr. 27, 1937 STATE PATENT Mahlman, Kenmore,

Niagara Falls, N. Y.,

and Walter D. Rossow, assignors, by mesne assignments, to TheCarborundum' Company,

Niagara Falls, N. Y.,

ware

a corporation of Dela- No Drawing; Application December 6, 1934, SerialNo. 756,329. In Canada June 20, 1932 w 11 Claims.

acteristics by using differing proportions of bond to abrasive grain inthe articles. While this method furnishes a means for securing differingcharacteristics in the grinding action of the bonded articles, it issomewhat limited in scope because the difference obtained by this meansis always accompanied by changes in other properties, such as strengthand particularly in the resistance to rupture from centrifugal force.Furthermore in order to obtain the so-called soft Q grinding article itis necessary to reduce the quantity 'of bond to such an extent that thearticles are materially weakened.

In the manufacture of ceramic bonded abrasive articles a furthervariation in grinding action beyond that obtained by changing theproportion of bond to grain is secured by altering the composition ofthe bond. Previously there has been no equivalent method of securing inorganic bonded abrasives the variability in grinding action which iscommon with ceramic bonded articles. The kinds of synthetic resinsavailable which are suitable for bonding abrasives are more or lesslimited and although there is some difference between abrasives bondedwith these different resins the difference is limited as compared withthat obtained with ceramic bonding compositions. This lack ofvariability in the action of resin bonded abrasives has materiallylimited the field in which such articles can be used. I 4 We havediscovered that the grinding action of resin bonded abrasives varieswith the coefficient of expanslonof the bond; and by bond is meant allof the finished article except the abrasive grain proper. We havefurther discovered that this variability is distinct from that securedby the previously available method of varying the proportion of bond tograin.

These phenomena appear to be due to certain factors which we will nowdiscuss by way of 5Q clarifying the understanding of the invention,without intendin however, to limit the invention by the theory. During agrinding operation the abrasive grains in contact with the work, becauseof that contact, become dulled, and if the abrasive 55 article is tocontinue to cut efiicientiy the dulled grain must be removed. Provisionfor this removal has been made in the past by so proportioning the bondto grain that the excess pressure built up between the abrasive article,for example a'wheel, and the work caused by the failure of the dulledgrain to remove the expected amount-of material is sufficient to rupturethe bond, thus breaking out the dull grain and exposing fresh sharpgrains at the surface of the wheel.

When a dull grain is repeatedly rubbed over a piece of work underthepressure of the grinding action the friction between the two surfacesresults in local heating as a result of which the adhesion between thebond and the grain is affected by the relative thermal expansion of thebopd and grain. This differential thermal expansion of the bond andgrain causes local stresses between the bond and grain which help tofree the grain from the bond. These'stresses in conjunction with thepressure between grain and i work resultin rupturing the grain-bondadhesion and breaking the grain loose.

Since the forces which break the grain from the bond include from thatof differential expansion of the grain and bond as well as the shock ofthe contact of the grains with the'work, it is possible to vary thetenacity of bond to grain over a greater range than that feasible whererupture depends upon pressure alone. For example, by

selectively varying the coefficient of expansion of the organic bond, weare able to amplify the force resulting from differential expansionbetween grain and bond to assist the breakdown of the wheel or thedifferential expansion can be reduced to a minimum thereby strengtheningthe wheel against the breakdown ordinarily due to local heating.

The present invention provides for varying the coefficient of expansionof the curedorganic bond, for example resin, inthe finished article withref erence to the thermal coeflicient of expansion of the abrasive grainby adding to the resin used in the mix for making the article acomminuted material having a relatively low thermal coemcient ofexpansion.

The coefficient of expansion of a resin bond material may be loweredwith advantage by the addition of comminuted materials having a ther malcoeificient of expansion lower than that of the abrasive grain used.This procedure is particularly advantageous as it reduces to a minimumthe modifier necessary to add to the resin to vary its coefficient ofexpansion withrespect to that of the abrasive grain.

Materials which have been found useful in comminuted form as modifiersof the resin bond are fused quartz, silicon carbide and certaindevitrified silicates of metals of the second group of the periodicsystem of the chemical elements as set up by Mendelefis law, such asmagnesium aluminosilicate.

The modifier may be added to the mix in any convenient manner to provideintimate mixture with the resin when cured. For example, when the grainis wetted with a plasticizer such as furfural and powdered potentiallyreactive resin is mixed with the wetted grain, the comminuted modifierwhich may comprise a plurality of the substances mentioned above, may beintimately mixed with the dry resin previous to its addition to the mix.

For example, a pressed cured.mixture of 400 grams of dry powdered phenolformaldehyde resin and 50 00.01 furfural had a coeificient of expansionof 39.22 10- C. Aluminum oxide abrasive grain has a thermal coefficientof expansion of 5.94 X 10- at 20-90 C. 1

Replacing 40 grams of the resin with 106 grams of pulverized fusedsilica and pressing and curing the mixture under the same conditionsproduced a piece having a coefficient of expansion of 32.72 10 C.

Replacing grams of the original mix with 265 grams of fused silica asabove and pressing and curing under the same conditions produced a piecehaving a coefficient of expansion of 24.7 i0 C.

When fused alumina or other abrasive having an equal or greatercoefficient of expansion is used it is convenient to use 'a modifiercomprising pulverized silicon carbide. For example, when 40 grams of thedry resin of the original mix set forth above were replaced with gramsof pulverized silicon carbide and the mix pressed and cured under thesame conditions as in the other examples, the piece was found to have acoeflicient of expansion of 31.62 10- C. a reduction of 7 points.

When 100 grams of the original 400 grams of dry powdered resin of theoriginal mix were replaced with 312 grams of pulverized silicon carbideit was found that the coefficient of expansion of the pressed and curedarticle had been reduced to 24.12 10- C.

Certain devitrified silicates of the metals of the second group of theperiodic system are particularly useful in connection with thisinvention because of their relatively low thermal coefiicient ofexpansion. These include magnesium alumino-silicate, zinc orthosilioate,barium alumino-silicate, beryllium alumino-silicate and calciumalumino-silicate. Of these the magnesium aIumino-silicate isparticularly useful because of its extremely low coefilcient ofexpansion and the ease with which it may be prepared. Its coefficient ofexpansion is of the order of magnitude of that of fused silica and mayapproach zero depending upon the presence or absence of certainimpurities.

The substitution of 106 grams of this magnesium alumino-silicate for 40grams of the dry powdered resin of the original example produced, whenthe mix was pressed and cured properly, an article having a coefficientof expansion of 28.62 10 C. An increase of the amount of modifier to 265grams of a corresponding decrease of the dry resin to 300 grams underthese same conditions produced an article having a thermal coefiicientof expansion of 24.62 l0 C.

It is to be understood that the foregoi amples are given merely toillustrate the effect of the addition of these modifiers to the resin onthe thermal coefiicient of expansion of the bond as compared with thatof the grain. It will be apparent that changes in the specificcomposition, for example as to the resin bond used or the plasticizerused, or the proportions of the plasticizer, resin and modifier, willgive substantially any value of coeificient of bond expansion which maybe desired within reason. Furthermore, the modifiers mentioned above aremerely illustrative of a class of materials having the required lowcoeflicients of expansion and which can be intimately mixed with resinin the curing thereof without adversely affecting the bond chemically.

It is customary to make abrasive articles in series of grades, thegrades depending in previous practice, primarily upon the proportion ofthe bond tograin in the article. By means of the present invention weare able to establish a similar set of articles made according to ourinvention with the added feature that we are able to make several seriesof articles, the grades of any series being produced as usual bychanging the bond content. In this Way we can systematically cover avery broad range of grinding char acteristics, such as are now covered,much more narrowly by variation in proportion of bond to grain. Thus wehave broadenedthe range of resin bonded abrasive to include severalseries as well as the one series now in common use.

We claim:

1. An improvement in the method of producing abrasive articles ofdesired grinding characteristics from a mixture of abrasive grain andresin-containing bond which comprises mixing abrasive grain and resinmodified by the addition of a comminuted substance having a coefiicientof expansion less than 5.0 10- C. at 20-90 C. and lower than that ofthesaid abrasive grain and controlling the extent to which thecoefiicient of expansion of the cured bond is lowered by selectivelyvarying the proportion of the said substance employed for modifying theresin.

2. An improvement in the method of producing abrasive articles ofdesired grinding characteristics from a mixture of abrasive grain andresincontaining bond which comprises mixing abrasive grain and resinmodified by the addition of pulverized fused quartz and controlling theextent to which the coeflicient of expansion of the cured bond islowered by selectively varying the proportions of the said fused quartzin the bond.

3. An improvement in the method of producing abrasive articles ofdesired grinding characteris tics from a mixture of aluminum oxideabrasive grain and resin-containing bond which comprises mixing theabrasive grain and resin modified by the addition of pulverized siliconcarbide and controlling the extent to which the coefficient of expansionof the cured bond is lowered by selectively varying the proportion ofthe said silicon carbide employed for modifying the resin.

4. An improvement in the method of producing abrasive articles ofdesired grinding characteristics from a mixture of abrasive grain andresincontaining bond which comprises mixing abrasive grain and resinmodified by the addition of a com minuted devitrified silicate of ametal of the second group of the periodic system having a coefii cientof expansion less than 5.0 l0" C. at 20- 90 C. and lower than that ofthe said abrasive grain and controlling the extent to which thecoefficient of expansion of the cured bond is lowered by selectivelyvarying the proportion of the said substance employed for modifying theresin.

5. An improvement in the method of producing v abrasive articles or"desired grinding characteristics from a mixture of abrasive grain andresin containing bond which comprises mixing abrasive grain and resinmodified by the addition of comminuted devitrified magnesium alumino-silicate and controlling the extent to which the coefficient of expansionof the cured bond is lowered by selectively varying the proportion ofthe said substance employed for modifying the resin 6. An abrasivearticle comprising abiiasive grain and a heat hardened resin-containingbond modified by a comminuted substance having a coefficient ofexpansion less than 5.0 X 10- C. at

20-90 C. and lower than that of the said abrasive grain. 1

'7. An abrasive article comprising abrasive grain r and a heat hardenedresin-containing bond modified by pulverized fused quartz.

8. An abrasive article comprising abrasive grain and a heat-hardenedresin-containing bond modified by a comminuted devitrlfied silicate of amodified by pulverized devitrified magnesium' alumino silicate. 1

11. A resinous bonded abrasive article comprising abrasive grain and abond containing a (synthetic resin with which has been incorporated amodifier having a coefficient of thermal expansion less than 5.0 X 10*C. at 20-90 C. to lower the coflicient of expansion of the bond, saidbond having due to the incorporation of said modifier a thermalcoefficient of expansion less than seven times that of the said abrasivegrain.

i f RAYMOND c. BENNER.

OSBORNE L. MAHLMAN. WALTER D. ROSSOW.

